It is necessary in various situations to electronically monitor whether an electrical lamp (be it a filament bulb, light emitting diode—LED—or some other type of lamp) is functioning correctly. A field of particular importance to the inventor concerns “mute lamps”.
Mute lamps are used in conjunction with potentially hazardous machinery. Many machines have safety systems, such as automatic emergency shut-off arrangements, which need from time to time to be temporarily disabled (or “muted”) to enable the machine to carry out its proper function. Safety regulations allow for this muting of the safety system but require that for its duration a warning must be provided by illumination of the mute lamp. Furthermore a fail safe arrangement must be provided to ensure that if the lamp should fail then the muting is automatically prevented—i.e. the safety system is activated. Hence the functioning of the mute lamp must itself be automatically monitored.
A known safety arrangement utilising a mute lamp is illustrated in FIG. 1. A safety light curtain 100 is intended to detect the presence of a person in a hazardous area and is connected to a mute control unit 102 whose outputs 104 are led to a controller (not illustrated) capable of shutting down the associated machinery. The mute control unit 102 also receives inputs from sensors 106 and is connected to a mute lamp 108.
This type of arrangement is needed where the light curtain 100 can be triggered not only by a person in the hazardous zone (the response to which should be to shut down the machinery) but also by, for example, the intentional movement of products under manufacture through the zone (in which case the machinery must not be shut down if it is to perform its function). To make it possible for the machinery to continue functioning in the latter situation, the mute control unit is capable of muting the response to the light curtain in response to the mute sensors 106. When the sensors 106 are triggered, the mute control unit applies a voltage to the mute lamp 108 and, provided that it detects that the lamp is illuminated to provide a visible warning, causes muting of the response to the light curtain so that triggering of the curtain does not cause the machinery to be shut down. If, however, the mute control unit 102 detects that the lamp 108 is not illuminated then muting does not take place. The light curtain thus remains active if the mute lamp malfunctions.
Known circuitry for monitoring of mute lamps is incorporated in the mute control unit and uses a current sensing resistor connected in series with the lamp, voltage across the resistor being electronically monitored so that if it falls below a chosen threshold (indicating inadequate current flow for lamp function) or rises above a chosen threshold (indicating that the lamp's resistance has fallen excessively due to failure thereof) then a signal is generated to indicate lamp failure and so prevent muting of the safety system.
The mute control unit is typically somewhat bulky and is housed in a unit separate from the lamp itself.
It must be understood however that the invention is applicable to lights in general and not only to mute lights. There are many contexts in which it is desirable to provide, in a compact unit, a means of monitoring the functioning of a lamp.